So on our coldest day so far (19) my chevy codes out def tank problem. Speed limited in 99 miles. So on a guess I park it in the garage for a couple days and the code goes away. Just had a thought that maybe block heater would maybe also heat it. I truly doubt it because that would make some sense. I understand how tough it must be keeping some concoction of piss and water from freezing

Well there are level sensors in the tank that may not have been reading when the fluid was frozen. That's my only guess without knowing the exact code that was in the ecm. We have been replacing alot of Def heaters lately on the 15s and 16s. Sorry I can't be of more help.